"I remember seeing Cosmotheka doing a "set piece" a couple of times which involved a singer who was "planted" in the audience, and who, at a pre-arranged moment, started singing, seemingly spontaneously, with the performance on the stage. I thought it worked really well. In retrospect, I wonder if that was an old music hall device. "

Yes I saw them do that. I think it was Andrew Frank and the song was "When a Nightingale Sang in Berkley Square".